The present invention relates to an ignition semiconductor device, and in particular to an ignition semiconductor device applicable to an ignition-device system of an internal combustion engine for an automobile.
An ignition-device system of an internal combustion engine for an automobile includes a distributor-less ignition system comprising an ignition coil and an ignition semiconductor device mounted for each cylinder of the internal combustion engine. An ignition semiconductor device used in such a system comprises a switching device for turning on and off a primary current of the ignition coil.
The ignition semiconductor device provided for each cylinder is individually turned on and off by an engine control unit, but this on/off control may not operate properly if, for example, the engine stalls. In particular, if a continuous drive signal is applied to the switching device, a continuous current flows to the primary side to destroy or burn the ignition coil or uncontrollably explode a particular cylinder, causing the engine to vibrate abnormally.
A device for correcting such an abnormal operation is described, for example, in Japanese Patent Application Laid Open No. 8-28415. According to the technique described in this publication, a continuous-conduction prevention circuit is provided such that if current flows through the switching device for a predetermined period of time or longer, by means of a drive signal from the engine control unit, the drive signal input to the switching device is forcibly cut off to stop driving of the switching device. This prevents damages to the switching device and the ignition coil that may be caused by the continuous conduction.
Such an ignition semiconductor device also has a current-limiting circuit to restrain the drive signal sent to the switching device in order to prevent its destruction if an over-current is detected in the switching device.
According to the conventional ignition semiconductor device, the current-limiting circuit prevents over-current through the use of an output stage element, thus preventing thermal destruction of the ignition semiconductor device and the ignition coil, and the continuous-conduction prevention circuit turns off the drive signal and thus the output-stage element of the ignition semiconductor device if a drive signal is continuously applied for a fixed period of time or longer. In particular, however, the turn-off operation performed after the fixed period of time or longer has elapsed is performed at the same speed as a normal operation, so that a high voltage occurs at a secondary winding of the ignition coil, as in normal operation, to ignite a gasoline-air mixture remaining in the cylinder, thereby applying an abnormal rotational force to the engine.
The present invention is provided in view of these points, and it is an object of the invention to provide an ignition semiconductor device that turns off an output-stage terminal after a drive signal has been continuously applied, wherein an unwanted high voltage is prevented from being generated in a second winding of an ignition coil during the turn-off operation.
To attain the above object, the present invention provides an ignition semiconductor device comprising a switching device connected in series with an ignition coil to controllably turn on or off a current flowing through the ignition coil, a current-limiting device for controlling the switching device so as to limit the current flowing through the ignition coil, and a voltage-limiting circuit for clamping a voltage emitted from the ignition coil. The ignition semiconductor device includes a timer circuit that starts operation in response to an input signal applied to a drive terminal of the switching device and outputs an output signal after a fixed period of time since the input signal has been applied, and a main-current gradual-reduction circuit that operates in response to an output signal from the timer circuit to reduce the current flowing through the switching device despite continuous application of the input signal.
According to such an ignition semiconductor device, if an input signal for turning the switching device on is continuously applied, the timer circuit outputs the output signal and, in response to the output signal, the main-current gradual-reduction circuit reduces the current flowing through the switching device. Consequently, the switching device is turned off more slowly than in the normal operation to cut off the current flowing through a primary winding of the ignition coil at a low speed, thereby restraining a high voltage from being generated in a secondary winding.